A standard computer-controlled laser imaging system has a laser and lens for emitting a laser-light beam and an acoustic/optical modulator for receiving and modulating the beam into a beam of first-order diffraction corresponding to the desired image. A first telescopic lens system and a rotary polygonal mirror deflect the modulated beam in the x-direction of, that is parallel to , the line to be written and a second telescopic lens system and a projecting lens receive the deflected main beam and cast same on the image surface.
In such arrangements is standard to deflect in the y-direction perpendicular to the x-direction, which is parallel to the line or row being printed, by means of an oscillating mirror, as opposed to the rotary polygonal mirror used for deflection in the x-direction. As a result the start of actual writing must be accurately established. The most modern system operates in that when the main laser is off an auxiliary laser directs a beam through the first optical system to the polygonal mirror. The secondary-laser beam scans over a plate with an optical raster whose lines run in the vertical y-direction. The intensity modulation created by these lines is detected by a photodetector lying outside the path of the main imaging beam. This photodetector is set up such that the polygonal beam is in just the right position for deflecting the imaging beam at the start of a line on the image surface when the auxiliary laser beam is detected by the photodetector, and then the start signal for the imaging laser beam is emitted. If necessary a time delay can be imposed between the photodetector and the starting circuit for the imaging laser beam. The provision of the second laser and the raster plate makes this system fairly complex and expensive.
In U.S. Pat. No. 4,337,994 a system is described wherein instead of a polygonal mirror use is made of a so-called hologon disk. A mirror is arranged at a position along the main imaging beam downstream of the hologon disk to intercept this beam when it is not on the image surface. A photodetector is associated with this mirror which responds to the beam when it is intercepted and is connected to circuitry to cut off the laser beam when this happens. A time circuit turns the laser beam on again after an interval corresponding to its return to the imaging surface. Such an arrangement is not usable for laser imaging systems which are provided with a polygonal mirror because errors created by the unavoidable imperfections of the mirror surface cannot be compensated for.